1. Field of the Invention:
The present invention relates to gettering semiconductor wafers by directing a laser beam and more specifically, to gettering a silicon wafer by introducing strain fields in the back surface thereof with a laser directed thereon to induce crystalline defects which trap contaminants such as heavy metals.
2. Description of the Related Art:
Strain fields existing on the back surface of a semiconductor substrate induce crystalline defects such as stacking faults and dislocations in a heat-treatment. Contaminants such as heavy metals at the upper surface of the substrate where various semiconductor elements are to be formed are trapped by the crystalline defects during the same heat-treatment by the so-called gettering effect. As a result, production yields of semiconductor devices are enhanced. This gettering technique has been widely used in manufacture of semiconductor devices.
It has been proposed in the U.S. Pat. No. 4,131,487 and in Journal of the Electrochemical Society, Vol. 128, No. 9, 1981, pp. 1975-1980 to use an Nd:YAG laser beam for introducing strain fields in the back surface of a semiconductor wafer. The back surface of the wafer is irradiated with a laser beam to melt a surface layer of the back surface which is then solidified to generate crystalline defects such as dislocation loops and getter heavy metals and the like. Strain fields can be introduced in the back surface of the wafer either before or during transistor elements are formed in the upper surface of the wafer.
The present inventors have found disadvantages of the proposed laser gettering method. The laser beam emitted from the Nd:YAG laser has a wavelength of 1.06 .mu.m which lies in infrared region at which the semiconductor substrate, particuarly, the silicon substrate, has a small absorption coefficient. In order to introduce the distortion field in the back surface of the semiconductor substrate to the extent that it may exhibit gettering ability, the energy density of the Nd:YAG laser must be increased. When irradiated with the Nd:YAG laser beam having such a high energy density, however, the scars of melt are nonuniformly and deeply formed in the semiconductor substrate to depths of as great as 50 .mu.m or more, causing the semiconductor substrate to be warped remarkably. With heat-treatments being repeated in the process of manufacturing semiconductor devices, furthermore, cracks often develop in the semiconductor substrate by the nonuniformly and deeply formed scars of melt. Furthermore, the Nd:YAG laser has such a small beam spot that the semiconductor wafer must be irradiated over an extended period of time for introducing distortion field throughout the whole back surface of the semiconductor wafer, resulting in a poor productivity.